The present invention relates to vertical polarization antennas having a vertical support structure of elongate shape and dipoles situated at various levels along the structure and coupled to a coaxial feeder cable.
The document WO 97/45892 discloses an antenna of the above kind, in particular an antenna that is omnidirectional in azimuth, having at least two dipoles situated at the same level on the vertical structure. The dipoles and the associated feeder arrays from the coaxial cable are integrated into the structure and constitute a symmetrical system.
The structure of this prior art antenna consists of two identical metal sections which are parallel to each other and at a clearly defined distance from each other and are therefore assembled by means of insulative seals. The sections each have a central part in the form of a longitudinal trough and two opposed and plane lateral branches. They are assembled together back-to-back with the two troughs facing outward and their plane branches disposed face-to-face in pairs and separated by the defined distance between the sections.
The dipoles and their associated feeder arrays are machined into the plane branches of the two sections, the two arms of each dipole being machined into two branches assembled together face-to-face and being suitably bent to constitute the dipole when the two sections are assembled together.
The coaxial feeder cable is terminated at one end of the above structure. Its outer conductor is electrically connected to a first of the two sections. Its inner conductor is electrically connected to an auxiliary conductor mounted in the trough of the same first section and held at a distance from the walls of the trough to reconstitute the coaxial feeder structure. It extends in this way to half the height of the first section, at which point it is connected to the second section by a coaxial connector, to enable symmetrical feeding of the dipoles situated on either side of the median transverse plane of the antenna structure, forming a symmetricalizer system.
The above kind of antenna generates a vertically polarized signal, which is omnidirectional in azimuth if the antenna includes at least two dipoles at each vertical level. It generates a vertically polarized signal which is directional in azimuth if the antenna includes only one dipole at each vertical level.
The vertically polarized azimuth signal from the above antenna also has a crossed polarization component which is horizontal and inherent to vertical polarization antennas with dipoles and is due in particular to the radiation from the horizontal metal parts of the antenna. This horizontal component of polarization, although weaker than the main vertical component and generally at a level of the order of 12 to 15 dB below the latter, is undesirable in itself because it may interfere with other types of antenna nearby.
The prior art antenna previously cited is also relatively complex and costly to make. Furthermore, it can be used only at low and medium frequencies because of its mechanical construction.
An object of the present invention is to provide a vertical polarization antenna of simple design and whose design minimizes the crossed horizontal polarization component. Another object of the present invention is to enable the antenna to be made either as a mechanical structure or as a printed circuit, for use at low and medium frequencies or at microwave frequencies.
The present invention provides a vertical polarization antenna including a vertical support structure of elongate shape and dipoles situated at different levels along said structure and coupled to a coaxial feeder cable, characterized in that it includes only one of said dipoles per level and in that said dipoles are coplanar and substantially colinear but inverted with respect to each other on a face of the antenna referred to as the front face.
Said antenna can also have at least one of the following additional features:
Said dipoles are divided into two groups and inverted from one group to the other.
The antenna includes a ground plane disposed laterally relative to each group of dipoles and coplanar therewith and a feeder line disposed laterally relative to each group of dipoles and situated in a plane separate from but parallel to the ground plane, said ground plane and said feeder line each having a first section on a first side and along one of said groups of dipoles, a second section on the other side and along the other group, and a median continuity section passing between the two groups, and further includes projections provided on said first and second sections of said ground plane and said feeder line and connected to said dipoles.
Said coaxial feeder cable extends against said ground plane as far as a mid-point of the median section of said ground plane and is connected to a corresponding mid-point of the median section of said feeder line by a coaxial output provided between said mid-points of said ground plane and said feeder line.
The antenna includes a reflector associated with said dipoles and mounted to face one face of said structure which is opposite that forming said front face of the antenna.